Māṇḍūkī Śikṣā 88

88

pādādau ca padādau ca saṃyogāvagraheṣu ca
jaḥ śabda iti vijñeyo yonyaḥ sa ya iti smṛtaḥ

At the beginning of a verse foot and at the beginning of a word, and in consonant clusters and avagrahas, / the j-sound is to be recognized, and that y which comes from its source is so recorded.